Musar for Numbers 23:7

Each day the wicked Bilaam would come to them to learn the impure names and other forms of sorcery. This is the meaning of the verse, “Balak summoned me from Aram, from the Mountains of the Ancient Time (Bamidbar 23:7), and the verse, “Thus says the man who perceives the vision of the Almighty, the One Who Falls and the One with Open Eyes” (ibid. 3-4). Before Bilaam was able to study sorcery he was first obliged to defile himself through all sorts of impure acts such as having a carnal relationship with his donkey, for such is the custom of the students of sorcery. They must defile themselves and lie with an animal. That is why the Torah places side by side the injunctions, “Whoever lies [carnally] with an animal shall die,” and, “You shall not allow a sorceress to live” (Shemos 22:17-18). For the power of sorcery emanates entirely from the great sources of impurity, Heaven spare us. To begin with, whoever wishes to go to the Mountains of Darkness must prepare incense to offer before Uza and Aza’el so that he may accept them as divinities. At the same time he must abandon the service of Hashem and relinquish all portion in holiness. He must subjugate and indenture himself entirely to the Sitrah Acharah. These are the things that he must accept upon himself when he offers the incense.